Engines that operate on multiple fuels are being developed. The engines may operate on mixtures including gasoline/ethanol, gasoline/compressed natural gas (CNG), diesel/gasoline, or diesel/ethanol. Each of these fuels may be stored in a separate fuel tank on board a vehicle.
It also has been proposed to separate fuels that have been combined into a single fuel mixture for the purpose of improving vehicle performance and fuel economy. One fuel mixture that may be separated into its individual component fuels is a gasoline\ethanol fuel mixture such as E10 (90% gasoline and 10% ethanol), E20, or E85. Furthermore, gasoline may be separated into a low-octane component gasoline and a high-octane component gasoline on board the vehicle. The separated fuels or fuel components may be stored in separate fuel tanks on board the vehicle.
U.S. Patent Applications 2008/0006333 A1 and 2010/0229966 A1 describes fuel systems that include multiple fuel tanks for storing different types of fuels. Fuel vapors from the multiple fuel tanks are routed to a single fuel vapor storage canister to limit airborne emissions. However, it may be more difficult to control an engine air-fuel ratio with such a system since a wider range of fuel vapors may be stored in the fuel vapor storage canister because of differences between fuels stored in the multiple fuel tanks. Further, the inventors herein have recognized that the fuel vapors from one tank may be reabsorbed into other fuel tanks in the fuel system. Re-absorption of separated fuel components into fuel tanks may change fuel properties in each of the multiple fuel tanks. If re-absorption were to occur, re-separating fuel from the fuel tanks may result in increased energy consumption, or the engine may be operated less efficiently to use the combined fuel that includes fuel from different fuel tanks.
The inventors herein have recognized that fuels having different properties may be stored in separate fuel tanks to leverage desirable properties of the different fuels. One important property is that the fuels will generate vapors, with each vapor having unique properties, including, but not limited to, octane ratings or air-fuel ratios. By having separate fuel vapor storage canisters in fluidic communication with each fuel tank, each fuel vapor storage canister may also contain fuel vapors having unique properties, including, but not limited to, octane ratings and air-fuel ratio. Additionally, the fuel vapor properties of fuel stored in a fuel vapor storage canisters may be the same as that of fuel vapors held in the fuel tank that is in fluidic communication with the fuel vapor storage canister. As such, the inventors have devised engine operating and purge controls to take advantage of the different fuel vapor properties of different fuel tanks and different fuel vapor storage canisters.
In one example described herein, the inventors have provided control over multiple vapor purge flows into the engine from multiple fuel vapor storage devices that are in fluidic communication with a respective, but equal number of multiple fuel tanks; each of the multiple fuel vapor purge flows are controlled to be in a same proportion of total fuel vapors purged as the proportion of liquid fuel of the total liquid fuels supplied from the respective fuel tanks to the engine. This novel type of control allows the unique properties of each fuel to be fully utilized, both as a liquid and as a vapor. For example, the high-octane vapor phase fuel may be purged into the engine in proportion to the high-octane liquid phase fuel currently being injected into the engine. Otherwise, by mixing vapors from each fuel tank together in a single canister, as shown in prior approaches, the advantage of the high-octane vapor phase fuel may not be realized. Additionally, by purging fuel vapors proportionate to use of similar liquid phase fuel, engine air-fuel ratio disturbances may be reduced since the overall stoichiometric fuel ratio remains constant. In this way, fuel properties may be leveraged to benefit engine operation. Further, engine air-fuel ratio control during fuel vapor purging of multiple fuel types may be improved.
The present description may provide several advantages. In particular, the approach may allow stored fuel vapors to be used to improve engine operation in a similar way that one liquid fuel may be used to improve engine performance over a different liquid fuel. Further, the approach may improve engine air-fuel ratio control by allowing a stoichiometric air-fuel ratio to remain constant. Further still, the approach may reduce the possibility of separated fuels from being reabsorbed into fuel having different properties.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.